1. Field of the Invention
Embodiments of the present invention generally relate to virtual boundary processing in adaptive loop filtering in video coding.
2. Description of the Related Art
Video compression, i.e., video coding, is an essential enabler for digital video products as it enables the storage and transmission of digital video. In general, video compression techniques apply prediction, transformation, quantization, and entropy coding to sequential blocks of pixels in a video sequence to compress, i.e., encode, the video sequence. Video decompression techniques generally perform the inverse of these operations in reverse order to decompress, i.e., decode, a compressed video sequence.
The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T WP3/16 and ISO/IEC JTC 1/SC 29/WG 11 is currently developing the next-generation video coding standard referred to as High Efficiency Video Coding (HEVC). HEVC is expected to provide around 50% improvement in coding efficiency over the current standard, H.264/AVC, as well as larger resolutions and higher frame rates. To address these requirements, HEVC utilizes larger block sizes than H.264/AVC. In HEVC, the largest coding unit (LCU) can be up to 64×64 in size, while in H.264/AVC, the macroblock size is fixed at 16×16.
Adaptive loop filtering (ALF) is a new coding tool proposed for HEVC. In general, ALF is an adaptive Wiener filtering technique applied after the deblocking filter to improve the reference picture used for encoding/decoding of subsequent pictures. The original ALF concept is explained in more detail in Y. Chiu and L. Xu, “Adaptive (Wiener) Filter for Video Compression,” ITU-T SG16 Contribution, C437, Geneva, CH, April 2008. As originally proposed, ALF used square filters and was carried out on entire deblocked pictures. Subsequently, block-based adaptive loop filtering was proposed in which ALF could be enabled and disabled on a block, i.e., coding unit, basis. In block-based ALF, the encoder signals to the decoder the map of blocks of a deblocked picture on which ALF is to be applied. Block-based ALF is described in more detail in T. Chujoh, et al., “Block-based Adaptive Loop Filter,” ITU-T SG16 Q.6 Document, VCEG-A118, Berlin, Del., July 2008.
A further refinement to block-based ALF, quadtree adaptive loop filtering, was subsequently proposed in which the map of blocks was signaled using a quadtree. Quad-tree ALF is described in more detail in T. Chujoh, et al., “Quadtree-based Adaptive Loop Filter,” ITU-T SG16 Contribution, C181, January 2009. The use of diamond shaped rather than square shaped ALF filters was then proposed to reduce computational complexity. Diamond shaped ALF filters for luma components are described in more detail in M. Karczewicz, et al., “A Hybrid Video Coder Based on Extended Macroblock Sizes, Improved Interpolation, and Flexible Motion Representation,” IEEE Trans. on Circuits and Systems for Video Technology, pp. 1698-1708, Vol. 20, No. 12, December 2010.